It comes down to timing. I've heard that there is a "timing error" or something associated w/ needle swing for the zz stitch, and that exacerbates the skipped stitches occurring on zz vs. straight stitches.
There seems to be a lot of problem with this, across the board, on all zig zag machines. That makes me think it's some sloppy quality control on the needles or the threads or both. Your mechanic skills are more advanced than mine, but I'll give it a shot.
What I have seen suggested:
- Adjust the timing on machines, which you did.
- Try different brand of needle or type of because there seem to be slight differences in the length that can make one brand work better or worse on any given machine. (Needle length "just right" = good timing vs. "timing is off.") And also the piercing power of a needles and machines varies, so you have to "mate" those two things so that the there's not a delay in when the thread loop being formed.
- Try a small or a larger needle. Sometimes the opposite of what you expect works.
- Fabric feeding rate has to be right: Adjust presser bar pressure to keep fabric from rising up. Or loosen it, just for giggles.
- Change the brand of thread. Or go thicker or thinner.
- Use silicone spray or Sewer's Aid on the needle to make it slicker.
- Spray thread with Static Guard b/c some threads apparently either develop static electricity or heat (they melt going through the needle?), and then the thread drags, and throws off the timing of the loop enough to mess with the stitches.
- Keep lint out of the tension assembly and bobbin case.
- Thread delivery system, too. Inspect the front of the machine for work parts or a burr or notch in the actual front side and the edge of the machine where the thread descends toward the needle. Some machines wind up with a groove carved in them from years of use. Someone said bend the thread guides a bit so they stand out enough to keep the thread up off the surface of the machine worked.